Upward continuation and polynomial trend analysis as a gravity data decomposition, case study at Ziway-Shala basin, central Main Ethiopian rift

Abstract

The first task in quantitative interpretation of a gravity data is separation of the Bouguer anomaly into its regional and residual components which are respectively related to deep and shallow subsurface geology. The decomposition process is subjective and non-unique as there is no single best approach to approximate the low frequency signature. For example, the use of spectral analysis and upward continuation require the wise choice of slope change location and continuation height respectively, which could be chosen differently by different researchers. This requires a need to work on more than one method and select the best to be applied for a given study area. The "best" choice is made based on the anomaly signature of the underlying geology. In this research, the most frequently used methods such as upward continuation and trend surface analysis methods are used and compared to approximate the regional field in Central Main Ethiopian rift bounded between 38 0 00 0 -39 0 30 0 E and 7 0 00 0 -8 0 30 0 N. The upward continuation height and the order of trend polynomial surface are first chosen, to approximate the regional gravity field signal. Accordingly, an upward continuation height of 6km and first order polynomial trend surface are chosen to be appropriate. Comparison of the two methods shows that the upward continuation technique reflects the shallow source anomalies of the area better than that of the first order linear trend surface. This outcome is verified against the result obtained based on the first vertical derivative method, spectral analysis depth estimation method, well-log data and surface geology of the area. It is therefore recommended to consider the various existing filtering techniques and choose the best candidate for the separation of the regional and residual components of the observed field.

Depth estimates of anomalous subsurface sources using 2D/3D modeling of potential field data: implications for groundwater dynamics in the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift

“…The Bouguer gravity anomalies are the sum of the effect of shallow (high frequency) and deep (low frequency) causative sources which need to be separated into their respective components. The low frequency (regional) component is approximated by 6 km upward continuation of the Bouguer gravity anomalies ( Kebede et al., 2020 ) ( Figure 4 b) and the corresponding residual gravity anomaly component ( Figure 4 c) is calculated by subtracting the estimated regional ( Figure 4 b) from the gridded Bouguer gravity anomalies map ( Figure 4 a). The estimated residual gravity anomalies are subjected to source depths estimation for the region under study.…”

“…The method is a statistical estimation ( Spector and Grant, 1970 ) based on low and high frequency anomalous bodies. These anomalies respectively categorized as sources of low, intermediate and high frequencies characteristics ( Kebede et al., 2020 ).…”

“…In geological section described in section 2.1.3 , there is no Mesozoic and crystalline basement surface till depth of 2.5 km ( Cherkose and Mizunaga, 2018 ). The gravity data analysis in the study area show that the crystalline basement horizon is found at approximate mean depth of 3.0 km ( Kebede et al., 2020 ). In-between Tertiary ignimbrite and crystalline basement layers, however, there is Mesozoic layer ( Woldegabriel et al., 2000 ).…”

“…These structures could be mapped using gravity and magnetic anomalies through the application of different mathematical filtering techniques. The subsurface relief for example could be mapped by defining density/susceptibility interfaces (source-depths) using filters like upward continuation ( Kebede et al, 2020 ), tilt depth ( Chen et al, 2016 ; Salem et al, 2007 ), 3D Euler Deconvolution ( Mammo, 2010 ; Keating and Pilkington, 2004 ), power spectral analysis ( Mammo, 2012 ), 2D Werner Deconvolution ( Mammo, 2012 ), 2D forward modeling ( Mickus et al, 2007 ), 3D structural inversion ( Tiberi et al, 2005 ), genetic algorithm ( Montesinos et al, 2016 ) and artificial neural network approach ( Alimoradi et al, 2011 ). The above mentioned source depth estimations methods are some of the various methods and are chosen differently by different researchers implying that there is no single approach.…”

“…Lake Shala gets its surface inflow from two main sources that enter from the southeastern and western rift escarpments ( Le Turdu et al., 1999 ).

Figure 3 Location of Ethiopia within the horn of Africa divided by the East African rift modified from ( Kebede et al., 2020 ) (a) Location and digital elevation map ( “USGS EROS Archive - Digital Elevation - Shuttle Radar Topography Mission (SRTM),” n.d. ) of the Ziway-Shala lakes Basin and its surroundings (b) Geological map modified from ( Kebede et al., 2020 ) and structural map overlaid geological map modified from Agostini et al. (2011) and Molin and Corti (2015) (c) Hydrological network of the Ziway-Shala Lakes Basin of its river network downloaded from ( EthioGis, n.d. ) (d).

…”